Automotive radar solutions for advanced driver assistance systems (ADAS) are currently being deployed on a large scale. These solutions can typically be grouped into long range radar (LRR) applications and short range radar (SRR) applications. Both of these applications generally use frequency modulated continuous wave (FMCW) modulation techniques in order to be able to identify a radar target, such as a car or a pedestrian.
These radar systems typically utilise millimeter wave (MMW) frequencies for transmission and reception. The frequency synthesisers, comprising voltage controlled oscillators (VCOs) that are responsible for the generation of the millimeter wave frequencies are important to the operation of the radar systems. Generally, voltage controlled oscillators operating at millimeter wave frequencies need to present a low phase noise, whilst providing a wide tuning range in order to cover the required modulation band (e.g. 1 GHz for LRR and 4 GHz for SRR).
Voltage controlled oscillators operating at millimeter wave frequencies generally suffer from centre frequency variation over extreme corners (process) and temperature conditions. Such centre frequency variations tend to reduce the available tuning range of these VCOs, which can limit the modulation bandwidth, thereby resulting in increased manufacturing yield losses.
In common VCO implementations, the output oscillation frequency is dependent upon capacitive, C, and inductive, L, element values within a resonator circuit. In most VCO designs, process and temperature variations shift the required oscillation frequency, fo, to a different value, which may not be a desired or acceptable frequency.
A common technique to re-centre the VCO oscillation frequency is to change the value of a capacitive element within the resonator circuit, which in turn re-tunes the oscillation frequency of the resonator circuit.
Referring to FIG. 1, a known device described in WO2014/006439, having an LC oscillator circuit controlled by a full varactor based capacitive arrangement, is illustrated. The LC oscillator circuit 100 comprises a pair of cross-coupled PMOS transistors, M1/M2, 102 that provide a negative resistance to an LC circuit that comprises coils 105 and a full varactor-based arrangement, for generating the oscillation.
The full varactor-based arrangement is composed of a coarse varactor bank 120 with 1 to ‘M’ identical varactors driven by a thermometric set of control signals, and a fine varactor bank 130. In operation, 1 to ‘M’ varactors can be selected in order to obtain a given coarse frequency step, as required by the modulation scheme employed by the device.
The fine varactor bank 130 is a full varactor-based capacitive divider, and has a main varactor bank in parallel with two shunt varactor banks, further series connected to two series varactor banks. By switching one varactor unit of the main varactor bank at a time, an equivalent capacitance step is created between the differential output nodes of the VCO 100, thereby enabling a required frequency change to be achieved. The shunt and series varactor banks are both controllable, enabling a further tuning of the achievable frequency resolution.
However, such MOS-based solutions are not suited to MMW VCOs for radar applications, due primarily to limited VCO tuning range and too high VCO phase noise. MOS-based varactor devices also suffer from a limited operating supply voltage when compared to bipolar-based devices, for example, and typically present a higher ‘off’ capacitance. This higher ‘off’ capacitance can be a limitation at MMW frequencies, for example above 20 GHz, since a high ‘off’ capacitance can reduce VCO tuning range, and therefore the overall operating frequency of the whole synthesizer.
A further limitation of the device in FIG. 1 is the frequency resolution (i.e. frequency steps or unit capacitance steps) imposed by the circuit construction, which are not constant when several elements are switched ‘on’.
Furthermore, MOS-based varactor devices generally present an intrinsically lower quality (Q) factor than, for example, bipolar-based varactor devices. This can make MOS-based varactor devices unsuitable for VCO systems that require stringent phase noise performance requirements, such as MMW radar applications.
In known VCO re-centering (tuning) operations, the traditional way to frequency (re-)tune in a phase locked loop (PLL) system is via band-selection, with the PLL locked in a closed loop mode of operation. However, as radar systems are FMCW based (where there is no concept of bands, channels, etc.), the VCO needs to be designed for a much wider tuning range than is required by the system. Thus far in such radar systems, as there has generally being a limited need for accuracy, the VCO re-centering (tuning) operation is performed in a closed loop mode of operation, with some coarse tuning of some capacitor banks located at the VCO tank.